1. Field of the Invention
The present invention relates to a focus detection device for a camera such as a single-lens reflex type camera or a video camera.
2. Related Background Art
A so-called differential phase type focus detection device for focusing a lens in accordance with a light distribution pattern of an image created by sensing light beams passing through different points on an exit pupil plane of a photographing lens by a photo-electric conversion device array such as a CCD has been widely used in a camera.
Since such a photo-electric conversion device array is usually arranged in a predetermined direction, for example, a horizontal direction which contains an optical axis at a center of a photographing field, it is difficult to detect a focus point of an object extending parallel to the photo-electric conversion device array, for example, a horizontal line. Further, such a device can detect the focus point only in the vicinity of the center of the photographing field.
To solve those problems, Japanese Laid-Open Patent Application No. 2-50115 proposes the following focus detection device. Namely, a plurality of photo-electric conversion device arrays are arranged. FIGS. 6A and 6B show focus detection areas on the photographing field. FIG. 5 shows a configuration of an optical system corresponding to FIG. 6A. Light beams transmitted through a photographing lens 211 are defined by a view field mask 222 having rectangular apertures 222a-222c for defining the view field, pass through condenser lenses 221a-221c, and are projected to CCD photo-electric conversion device arrays 216a-216c on a substrate 217 by an iris mask 219 and pairs of separator lenses 218. A configuration of an optical system for FIG. 6B is not shown but it is seen from FIG. 5 and other embodiments that refocusing optical systems for focus detection areas 2a.sub.2 -2e.sub.2 are required.
In the device disclosed in Japanese Laid-Open Patent Application No. 2-50115, off-axis areas 2b.sub.1 and 2c.sub.1 extend horizontally of the photographing field as shown in FIG. 6A. While the areas 2b.sub.1 and 2c.sub.1 are arranged on the same line, the distance therebetween is very large. Thus, independent refocusing optical systems are required as shown by condenser lenses 221b and 221c separator lenses 218b and 218c, and irises 219b and 219c. Where the off-axis areas 2b.sub.1 and 2c.sub.1 are to be focused by one refocusing optical system, focus detection light beams must be sharply deflected and various aberrations are included in refocusing the light beams on the CCD photo-electric conversion device array. It is very difficult from an optical property standpoint to solve the above problem.
FIG. 6B shows that off-axis areas 2b.sub.2 -2e.sub.2 are arranged vertically and horizontally. Since the CCD photo-electric conversion device arrays for those areas 2b.sub.2 -2e.sub.2 are not arranged on the same line, those four areas 2b.sub.2 -2e.sub.2 require four refocusing optical systems.
In such a device, the number of parts such as lenses (condenser lenses and refocusing lenses) increases, which causes the increase of cost and the increase of adjustment time of the optical systems.